(1) Field of the Invention
This invention relates to a reactive extrusion or blending process as a means of developing a new, formaldehyde-free binding system for wood composite products. The surfaces of dried wood particles were particularly modified by grafting anhydride modified polyolefin using reactive blending or extrusion. Chemical changes resulting from this treatment were followed the FTIR and XPS spectra. The modified wood particles were compression-molded into panels, in absence of a non-functionalized polyolefin, which were tested for physico-mechanical properties. Both FTIR and XPS data revealed that the chemical reactions have taken place between the hydroxyl groups of wood particles and anhydride modified polyolefin. The results showed that the performance of composite panels compared favorably with current standard requirements for particleboard.
(2) Description of Related Art
Wood-based composites are commonly made using formaldehyde-based adhesives, including urea-formaldehyde, melamine-formaldehyde, and phenol-formaldehyde (Maloney, T. M., Forest Prod. J., 46 (2): 19-26 (1996); Guss, L. M., Forest Prod. J., 45 (7/8): 17-24 (1995); and Sellers, T., Wood Technol. May/June Issue: pp. 40-43 (2000)). In 1998, 1,780 kilotons of adhesive resin solids were used to produce primary glued wood products (excluding the adhesive used to bond furniture and other secondary wood products). Of this amount, nearly 92% were formaldehyde-based adhesives (Sellers, T., Wood technol., May/June Issue: pp. 40-43 (2000)).
Plants that produce wood composites using formaldehyde-based adhesives emit harmful chemicals to the environment. These include phenol, formaldehyde, ketones, and other compounds, which are known hazardous air pollutants (HAPs) (Wang, W., et al., Forest Prod. J., 53(3):65-72 (2003); and Barry, A. et al., Forest Prod. J., 50(10):35-42 (2000)). Common composite products such as plywood, oriented strandboard and particleboard are used in building construction and in furniture, which is a concern as these products also tend to release formaldehyde over time (Maloney, T. M., Modern Particleboard and Dry Process Fiberboard Manufacturing. Updated edition. Miller Freeman, San Francisco, Calif. (1993)). Wood composites made today emit far less formaldehyde than those made 20 years ago, but the problem has not been eliminated. Formaldehyde and other toxic compounds may be present in large amounts in both indoor and outdoor air as a consequence of the use of these adhesives (Anonymous, “summary of working draft of proposed rule for plywood and composite wood products,” National Emission Standards for Hazardous Air Pollutants (NESHAP), Rule Development Project Lead: Greg Nizich (nizich.greg@epa.gov), U.S. Environmental Protection Agency (EPA), Technology Transfer Network-Air Toxics Website, August 2002, http://www.epa.gov/ttn/atw/plypart/plypart.html).
As a result of public concern about the environment, the Environmental Protection Agency (EPA) has enacted new rules for facilities that manufacture plywood and composite wood products in September 2004 (U.S. EPA, Federal Register, 69 (146): 45943-46046 (2004)). These rules affect both new and existing plants that produce at least 10 tons of any one HAP per year, or any combination of 25 tons of HAPs per year. The California Air Resources Board has gone even further; proposing a regulation that would eliminate urea-formaldehyde based wood composites from being sold in California, regardless of where they were made (Anonymous, “Fact sheet: Composite Wood Products,” California Air Resources Board Website, march 2003, http://www.arb.ca.gov). These regulations will force industry to find new ways to bind composite products without the use of formaldehyde-based adhesives.
In recent years, there have been several studies into environmentally friendly wood adhesives (Sellers, T., Wood Technol., May/June Issue: pp. 40-43 (2000)). Some of the areas that have been investigated include urea-formaldehyde adhesives with low formaldehyde-to-urea molar ratios, and the development of tannin, lignin, soybean and cornstarch adhesives (Sellers, T., Wood technol., May/June Issue: pp. 40-43 (2000); and Pizzi, A., Wood adhesives: Chemistry and Technology, Volume 1, edited by A. Pizzi, Marcel Dekker, New York (1989)), and phenol-formaldehyde resins modified with lignin (Matuana, L. M., et al., Eur. Polym. J., 29 (4): 483-490 (1993) and Kazayawoko, J. S. M., et al., Holzforschung, 46(3): 257-261(1992)). Additives that reduce formaldehyde release during composite pressing and during board use have also been developed and are currently in use (Pizzi, A., Wood Adhesives: Chemistry and Technology, Volume 1, edited by A. Pizzi, Marcel Dekker, New York (1989)). The regulation of formaldehyde emissions has lead to some development of fiberboard without synthetic adhesives (Velasquez, J. A., et al., Holz als Roh-und Werkstoff 60:297-302(2002); and Widsten, P., et al., Holzforschung 57:447-452 (2003)). The binderless boards and those made with natural adhesives tend to have poorer mechanical properties than those made with synthetic adhesives.
Prior work demonstrated the ability to graft maleated polyolefins to cellulose through a wet process (Li, Q., et al., J. Appl. Polym. Sci. 88: 278-286 (2003)). However, the wet process had the drawback of using organic solvents which had to be removed through drying. Therefore, the wet process is both expensive and time consuming on an industrial scale.